To date there has not been a great deal of activity in high power inverter circuits employing high power transistors, capable of switching 1,250 amperes at 400 volts to provide AC power to variable speed motors 350 horsepower and larger. In order to handle these high levels of power there has been developed a drive circuit for parallel non-matched transistors which allows for non-matched transistors to be utilized without concern that the transistor will burn out because of inherently different on-times. The inventor of the invention to be described hereinafter, namely Timothy Glennon, in his U.S. Pat. No. 4,356,408 issued Oct. 26, 1982, which patent shares a common assignee with this application, solved the transistor burn out problem which has lead to what is believed to be to date the highest power density inverter motor ever demonstrated.
Until now, a power density of one (1) horsepower per pound had been attained. The invention to be described hereinafter forms in part an essential feature of what has been demonstrated as a PWM variable speed electric motor having a power density of 2 horsepower per pound. In prior art PWM motor controlled systems, the technique of using a voltage controlled oscillator to generate a frequency in response to a voltage level is well known. Normally, a square wave results. It is also well known that if a square wave is applied to an integrator, a triangle wave results. Typically a triangle wave generated in the fashion just described is utilized as the modulation source for the PWM motor control unit. In those environments where the voltage level, i.e., V.sub.IN are proportional to a motor speed commanded, and the motor speed is relatively constant, few problems arise. However, where the motor speed commanded, ranges from zero to a maximum speed to provide maximum horsepower, in for example, a torpedo propulsion system, the triangle wave amplitude has been found to decrease directly as frequency increased. Since the triangle wave amplitude is directly proportional to loop gain in a pulse width modulated (PWM) control system of the prior art, the loop gain varied directly with speed. This is highly undesirable, and this undesirable loop gain and its affect on the error control of the PWM will be described more fully hereinafter.
The invention to be described hereinafter produces a constant amplitude variable frequency triangle wave for use in a motor control PWM system.
There have been previous efforts to provide variable frequency triangle waves that have more or less constant amplitude. Typical of such efforts is that of Dudley in his U.S. Pat. No. 3,440,448 which illustrates a specific circuit for producing symetrical triangle waves in response to a variable voltage input. Dudley therefore appears to be the functional equivalent from an input-output viewpoint of the invention to be described hereinafter.
The Dudley waveform generator is directed to a circuit for producing a symetrical triangular waveform of variable repetition rate which includes an integrator circuit 9, 11 connected to receive the currents from a pair of current sources which are switched at set levels of amplitude of the generated triangular wave. The integrator circuit 9, 11 has an output coupled to a comparator. The repetition rate of the triangular waveform is varied by altering the amplitude of both current sources in a fixed ratio. The invention to be described hereinafter does not require a comparator connected to the output of the integrator to cause the current source on the input of the integrator to reverse. The invention of this specification uses an off the shelf voltage controlled oscillator (VCO) coupled with a differential integrator to produce the constant amplitude triangle wave, and further provides that the fundamental frequency be established by the VCO, and not by the triangle wave generated as in Dudley.
Another pertinent patent is that of Chandos U.S. Pat. No. 3,610,952, which patent is directed to a device for automatically converting a square wave input signal into a constant amplitude triangular wave output signal regardless of changes in the amplitude or frequency of the input square wave signal. Chandos accomplishes this by providing a variable resistance device to receive a square wave input signal and transfer an input signal to a fast integrator circuit which converts the square wave signal into an integrator output triangular wave signal. A buffer stage removes any DC component from the integrator output triangular wave signal and buffers it into a triangular wave output signal. A signal-clipping means that may comprise a zener diode connected to the output of the buffer applied a clipped portion of the triangular wave output signal, together with a bias signal, to a slow integrator circuit. The slow integrator circuit signal, comprising a control signal, is responsive to the voltage differences between the zener diode-clipped signal and the fixed (but controllable) bias signal. The slow integrator output is coupled to control the variable resistance device in order to increase or decrease the time constant of the fast integrator circuit which varies the amplitude of the integrator output triangular wave signal and thereby maintains the triangular wave output signal at a constant amplitude.
The invention to be described in the specification hereinafter accomplishes similar goals as Chandos, but with a consumately simpler circuit arrangement which does not require such circuit components as "variable resistance means, fast integrator, slow integrator, etc".